A METHOD FOR FORMING AN ETCHED SOFT EDGE METAL FOIL

AND THE PRODUCT THEREOF

This patent application claims the benefit of priority of U.S. Provisional Patent Application Serial No. 60/699,927, filed July 14, 2005, entitled "METHOD FOR FORMING AN ETCHED SOFT EDGE TECHNOLOGY AND THE PRODUCT THEREOF" the entire disclosure of which is incorporated herein by reference.

Field of Invention

The invention relates generally to metal processing and more particularly, to the

processing of thin metal foils or metal strips.

Background of the Invention

Many industries require thin metal parts with precise dimensions. Such parts

include optical components, encoder discs, switch contacts, spacers and gaskets,

meshes, thin screens, flat springs, and foils. There are currently numerous methods

known to produce thin metal parts. The most popular methods are electroforming,

etching, punch and die stamping, and cutting, either by laser or water jet.

Electroforming is a process by which a metal part is produced on a base form. The

base form is negatively charged and placed in a solution containing a salt of the metal

desired for the part. The metal is deposited onto the base form and the part is formed.

In contrast, the other methods begin with a thin metal sheet and remove material until

the desired part is produced. Etching uses an acid to remove the base metal by

chemical or electro-chemical corrosion. An etch-resistant protective coating is adhered

to the object in areas where the metal should remain. During the etch, only the

unprotected parts are removed by corrosion.

For precision applications electroforming, etching, and punch and die stamping

have a clear advantage over cutting because heat or mechanical energy generated during

the cutting process deforms the edges of the cut. Electroforming and etching have a

further advantage over stamping because the stamping process cannot readily create

grooves or notches of a depth approximating half of the material thickness in the metal

part. Although electroforming and etching can both produce seemingly very similar

parts, the parts have geometries at a microscopic level that are quite different.

One application where thin metal parts are used is electric foil-head shavers. As

shown in Fig. 1, a foil-head shaver 1 has a cutting element 5 which oscillates under a

shaver foil 10. The shaver foil 10 serves as a guard to protect the user's face. The

shaver foil 10, as shown in Fig. 2, has a skin side 20, a cutter side 25 and a plurality of

whisker holes 30 which allow the user's hair to pass through and be cut by the cutting

element 5.

Conventionally, shaver foils are produced by electroforming of nickel or another

metal. A cross-sectional view of a shaver foil made by electroforming is shown in Fig.

3a. The cross section shown passes through the center of the whisker hole and includes

the thinnest part of the shaver foil between adjacent whisker holes. An advantage of

the electroformed foil is that the edges 56 of the whisker holes 30 on the face side 20 of

the shaver foil 10 are rounded, providing user comfort. On the other hand, the edges

51 of the whisker holes 30 on the cutter side 25 are blunt resulting in poor cuts. This

results in a shaver which is comfortable to use but does not perform optimally.

An alternative method of producing shaver foils is by etching the whisker holes

in a preexisting metal foil. Fig. 3b shows a cross section corresponding to 3a of a

shaver foil made by etching The etching produces sharp edges 57 and 52 on both the

face side 20 and the cutter side 25, respectively, of the shaver foil 10. The sharp edges

52 on the cutter side 25 improve the cutting ability of the shaver, but the sharp edges 57

on the face side 20 are uncomfortable for the user. The etching process also allows the

metal foil to be produced from various wrought metal alloys including stainless steels

which have more desirable properties than can be produced by the electroforming

process.

Summary of the Invention

The present invention is for a thin-metal processing method which removes

material from a thin metal foil or a metal strip. The processing method involves

etching to create removed areas in the metal foil or strip. Novel surface preparation

steps allow edges surrounding the removed areas to be either sharp or rounded. The

processing method further allows both the sharp and rounded edges to be made in a single etching pass.

The present invention also relates to an improved shaver foil for a foil-head

shaver. The shaver foil provides both face comfort and superior cutting ability over

current foils.

The shaver foil includes a skin side, a cutter side and a plurality of whisker

holes. The whisker holes allow a user's hair to pass through the shaver foil and be cut

by a cutting element while the shaver foil protects the user's skin. The whisker holes

have edges on both the face side and the skin side of the shaver foil. The skin side

edge is a rounded "soft-edge" which provides comfort for the user's face. The cutter

side edge is sharp to improve cutting ability.

The aforementioned thin-metal processing method is used to produce the

improved shaver foil and includes a plurality of steps. The plurality of steps include (i)

application of photo-resist or any other substance known in the art that resists corrosion

during etching to an unclean surface of a thin metal foil or metal strip where a soft edge

is desired, wherein the photo-resist or other corrosion resistant substance will

improperly adhere to the surface due to the presence of elements such as metal

particles, oils, small organic materials and/or oxidation on the unclean surface and the

adhesion will fail during an etching step, allowing a slight corrosion of the base

material to occur at areas where adhesion has failed; (ii) application of photo-resist or

any other substance known in the art that resists corrosion during etching to a clean

surface of a base material where a sharp edge is desired, wherein the photo-resist or

other corrosion resistant substance will optimally adhere to the surface and prevent

corrosion of the base material under that substance; and (iii) etching of the base

material to produce a foil with a number of openings, each opening having a sharp edge

on one side and a soft edge on another side.

Another metal processing method of the present invention includes the steps of

(i) preparing a metal surface to produce marginal adhesion between the metal surface

and a layer of etch-resistant material; (ii) applying the etch resistant material to the

metal surface; and (iii) etching the metal. During the etching step the adhesive bond is

progressively degraded with cumulative residence time in the etching acid, causing the

resist bond to fail and delaminate only at or near the end of the etching step. Only

limited edge rounding takes place locally on the immediate surfaces adjacent to the

perimeter of the etched part, rather than generally across the entire surface.

Brief Description of the Drawings

The foregoing and other features of the present invention will be more readily

apparent from the following detailed description and drawings of illustrative

embodiments of the invention in which:

Fig. 1 is a perspective view of a prior art foil-head shaver;

Fig. 2 is a perspective view of a prior art shaver foil;

Fig. 3a is an enlarged cross-sectional view of a single whisker hole within a

prior art shaver foil produced by electroforming;

Fig. 3b is an enlarged cross-sectional view of a single whisker hole within a

prior art shaver foil produced by etching;

Fig. 4 is an enlarged cross-sectional view of a single whisker hole within a

shaver foil produced in accordance with the present invention;

Fig. 5 is a bottom view of a shaver foil in accordance with the present

invention;

Fig. 6a is an enlarged cross-sectional view of a thin metal foil coated with etch

resistant material and a protective coating on one side in accordance with the present

invention;

Fig. 6b is an enlarged cross-sectional view of a thin metal foil coated with etch

resistant material and a protective coating on both sides in accordance with the present

invention;

Fig. 7a is an enlarged cross-sectional view of a thin metal foil coated with

partially exposed etch resistant material in accordance with the present invention;

Fig. 7b is an enlarged cross-sectional view of a thin metal foil with an attached

pattern of etch resistant material in accordance with the present invention; and

Fig. 8 is an enlarged cross-sectional view of a thin metal foil and an attached

pattern of etch resistant material after it has been etched in accordance with the present

invention.

Detailed Description of the Preferred Embodiments

As shown in Fig. 1, a conventional foil-head shaver 1 includes a shaver foil 10

and a cutter element 5. The shaver foil 10 shown in detail in Fig. 2 includes a skin side

20, a cutter side 25 and a plurality of openings or holes 30. The openings, or whisker

holes 30, allow a user's hair to pass through the shaver foil 10 and be cut by the cutter

element 5. Fig. 4 shows a cross-section of one whisker hole 30 in accordance with an

embodiment of the present invention, wherein the whisker hole is circular. The cross

section shown in Fig. 4 passes through the center of the circular whisker hole 30 and

shows the thinnest part of the shaver foil 10 between adjacent whisker holes 30. An

edge profile 40 defines the perimeter of the whisker hole 30 and extends from the skin

side 20 of the shaver foil 10 to the cutter side 25 of the shaver foil 10. A skin-side

edge 50 is formed where the edge profile 40 meets the skin side 20, and a cutter-side

edge 55 is formed where the edge profile 40 meets the cutter side 25. The edge profile

40 should be angled inward from the skin side 20 to the cutter side 25, such that the

skin-side circumference of the hole 30 is larger than the cutter-side circumference of

the hole 30. In other words, an angle 45 between the cutter side 25 of the shaver foil

10 and the edge profile 40 is acute. Thus, the cutter-side edge 55 is sharp. In contrast,

an angle 47 between the skin side 20 and the edge profile 40 is obtuse. Additionally,

the skin-side edge 50 is rounded.

The geometries of the skin-side edge 50 and the cutter-side edge 55 provide an

improved shaver foil 10 in contrast to conventional shaver foils. The rounded skin-side

edge is comfortable to the user's skin. There are no sharp edges which rub across the

user's face. In addition, the sharp cutter-side edge 55 provides superior cutting ability

over conventional shaver foils. When the sharp cutter-side edge 55 and the cutter

element 5 come into close contact, any hairs positioned therebetween have a higher

probability of being cut than if the cutter-side edge is blunt.

In a preferred embodiment, as shown in Fig. 5, a relief 60 is formed over the

majority of the cutter side 25 of the shaver foil 10. Only a margin 65 around the

whisker holes 30 protrudes. The relief 60 helps reduce the amount of material which

comes into close contact with the cutter. This reduces friction and the heat resulting

thereof.

The present invention also relates to a method for producing the shaver foils.

The preferred production method is described below in detail. However, the steps

presented are not required to be carried out in a particular order, nor are the specific

materials discussed the only materials which can be used to perform the method of the

present invention.

Surface preparation is an important aspect of the method used in the present

invention. Stock metal, as received from metal distributors, is never completely clean.

Although the metal may not appear "dirty," the surface is likely to contain at least one

contaminant. These contaminants may include organic materials, oils, small particles

of metal resulting from the stock metal processing and slight oxidation of the surface.

In order to produce an optimal adhesion between the etch-resistant coating and the

metal surface, all contaminants must be removed. If the contaminants are not removed,

the adhesion will fail during etching and the etch-resistant coating will not protect the

metal surface.

The present invention uses controlled adhesion failure between the metal surface

and the etch-resistant coating to create rounded edges at the boundaries of the etched

part. In contrast, conventional etch preparation with optimal adhesion between the

metal surface and the etch-resistant coating produces sharp edges around the etched

perimeter. An embodiment of the preparation and etching method in accordance with

the present invention is described in detail below.

A metal foil as received from a metal distributor is moderately cleaned. The

cleaning process used should remove only a fraction of the contaminants on the metal

surface. One embodiment of the moderate cleaning process in accordance with the

present invention uses a microetch to remove a fraction of the contaminants. The

microetch includes sulfuric and ferric chloride acids. In an alternative embodiment of

the moderate cleaning process, an acid wash is used to moderately clean the metal foil.

Regardless of the cleaning process used, some contaminants remain on the surface of

the metal foil. Alternatively, the surface of the stock metal could be deliberately contaminated as a pre-treatment step to assure desired response to an optional cleaning process.

As shown in Fig. 6, the moderately clean metal foil 100 has a first side 110 and

a second side 115 and is coated with a first layer of etch-resistant material (photo-resist)

120 on the first side 110. An imperfect adhesion between the first layer of photo-resist

120 and the first side 110 is made because of the remaining contaminants on the surface

110 of the metal foil 100. The first layer of photo-resist 120 is then covered with a

protective coating 128; polyester film is used for the protective coating 128 in a

preferred embodiment. The second side 115 is then properly cleaned. No harm is

done to the first layer of photo-resist 120 during the cleaning because of the protective

coating 128. As shown in Fig. 6b, a second layer of photo-resist 122 plus an additional

protective coating 128 is then applied to the second side 115 of the metal foil. An

optimal adhesion is formed between the second side 115 and the second layer of photo¬

resist 122.

Before etching the foil 100, sections of the photo-resist layers 120/122 are

removed to create a pattern of photo-resist 124 on the metal surfaces 110/115. This

will allow only some sections of the foil 100 to be removed during the etch. The

sections of the metal foil 100 that are protected by the photo-resist pattern 124 will not

be removed. The removed sections of photo-resist in the preferred embodiment include

larger circles on the first side 110 and smaller circles on the second side 115. This will

produce etched holes that angle inward from the first side 110 to the second side 115.

Various methods may be used to create the pattern of photo-resist 124 on the

foil 100. The methods depend on the type of photo-resist used. Either positive or

negative photo-resist may be used. Positive photo-resist becomes more soluble when

exposed to ultraviolet light. Any exposed sections of positive photo-resist are easily

removed from the applied surface in a solution. The unexposed sections remain on the

surface. In contrast, negative photo-resist polymerizes when exposed to ultraviolet

light causing the polymer to become more stable and prevents dissolution in developer

chemicals. The exposed sections of negative photo-resist remain on the surface while

the unexposed sections of negative photo-resist are removed in a solution. In both

methods, sections of the photo-resist layers 120/122 are exposed to ultraviolet light in

order to create the photo-resist pattern 124. There are various methods that may be

used to control which sections of photo-resist are exposed to the ultraviolet light. One

method uses a filter at the source of the ultraviolet light, to project a pattern onto the

photo-resist layers 120/122. An alternative method uses an opaque mask covering the

photo-resist layers 120/122 to block some of the ultraviolet light. The preferred

embodiment uses negative photo-resist and ultraviolet light that is filtered at the source.

Figs. 7a, 7b and 8 illustrate the preferred embodiment of the present invention.

As shown in Fig. 7a, both layers of photo-resist 120/122 are exposed to a

pattern of ultraviolet rays 140. The ultraviolet rays 140 produce a pattern of exposed

photo-resist 124 and a remainder of unexposed photo-resist 126. During the exposure,

the photo-resist polymerizes. The unexposed photo-resist 126 is then removed from the

foil 100 in a solution, and only the pattern of exposed photo-resist 124 remains. The

metal foil 100 with the attached pattern of exposed photo-resist 124 is shown in Fig.

7b. The boundaries of the pattern of exposed photo-resist 124 define first edges 152

where the exposed photo-resist 124 and metal foil 100 meet on the first side 110 and

second edges 153 where the exposed photo-resist 124 and metal foil 100 meet on the

second side 115.

The metal foil 100 is then subjected to an etching acid and holes 30 are formed.

In the preferred embodiment, high specific gravity ferric chloride etching acid is used.

The resulting part is shown in Fig. 8. During the etch, areas of the metal foil 100 that

were not protected by photo-resist are removed completely forming a plurality of holes

30 in the metal foil 100. The optimal adhesion between the pattern of photo-resist 124

on the second side 115 and the metal foil 100 keeps the photo-resist on the second side

115 intact during the etch. This produces a sharp edge 155 where the side 140 of the

hole 30 meets the second side 115 of the metal foil 100. In contrast, the adhesion

between the pattern of exposed photo-resist 124 on the first side 110 and the metal foil

100 fails during the etch. The failure begins at the first edges 152 of the photo-resist

pattern 124 on the first side 110. As the adhesion fails the areas under which failure

occurs come into contact with the etching acid. As a result, the etching acid corrodes

the metal in those areas and creates rounded edges 150.

In an alternative embodiment (not shown) the adhesion between the photo-resist

pattern 124 and the first side 110 of the metal foil 100 fails almost entirely during the

etch. The adhesion does not fail upon immediate contact with the etching acid, rather,

the photo-resist pattern is slowly peeled from the edges 152 until it is nearly entirely

removed. As a result, the areas of the metal foil 100 under the edges 152 of the photo¬

resist pattern 124 erode more than the areas under the majority of the photo-resist

pattern 124 and a gradual curve is formed from the edge of the hole 30 to the first side

110 of the metal foil 100.

After etching, the photo-resist polymer patterns are removed and the resulting

part is the shaver foil. The first side 110 is the skin side 20, the second side 115 is the

cutter side 25 and the holes 30 are the whisker holes 30 of the shaver foil 10.

In an alternative embodiment of the present invention the metal foil may be first

properly cleaned to remove all contaminants from the metal surface and photo-resist

may be applied to those areas where optimal adhesion is desired to produce sharp

edges. An adhesion inhibiting material may then be applied to the metal foil to inhibit

the adhesion of a second layer of photo-resist applied to the metal foil. This will result

in a failure of the adhesion and rounded "soft" edges will result. Alternatively, the

adhesion inhibiting material may be applied to the second layer of photo-resist before

application and both layers of photo-resist may be applied simultaneously.

Although the preferred form of the invention has been shown and described, many features may be varied, as will readily be apparent to those skilled in this art. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.